1. FIELD OF THE INVENTION
The present invention relates to a signal clamp system for a television receiver.
2. DESCRIPTION OF THE RELATED ART
A band compression transmission system called "MUSE" has been suggested by JP-A-60-86994 in which a video signal for High Vision (high-definition television system) for producing an image more detailed than the existing standard television systems is transmitted by one channel of 27 MHz bandwidth from a broadcasting satellite. For this television signal, a positive synchronization for including a sync signal within a video signal amplitude is employed to maximize the signal-to-noise (S/N) ratio. This prevents the use of a conventional method in which horizontal and vertical sync signals are separated from the television signal and the sync signals thus separated are used to drive a sync circuit of the television receiver. As a result, in the MUSE system mentioned above, for example, a vertical sync signal having a special form is inserted in the vertical flyback period and this signal is extracted by use of the correlationship between scanning lines, after which the horizontal sync signal is extracted thereby to attain the phase synchronization between an input television signal and a sync oscillation circuit.
This television signal, on the other hand, is comprised of a color difference signal compressed along the time axis and superimposed during the horizontal flyback period of the luminance signal and therefore has an extremely short pedestal period for clamp operation. Restoration of a band-compressed television signal requires a digital signal processing, which in turn makes it necessary to fix a DC potential in analog-digital conversion (hereinafter called A/D conversion) of the television signal. A clamp device is indispensable in fixing the DC potential. The horizontal clamp period of the television signal is so narrow that a signal with a very low S/N would be clamped at the formed end of a noise with the result that the noise would often cause the DC potential of the video signal to fluctuate. A vertical clamp system permitting a longer clamp period would be a solution to this problem. The vertical clamp, however, has a longer time constant for clamp and is often unable to compensate for sharp potential fluctuations of the television signal. While this may not pose any problem in the case of satellite broadcasting, it may pose problems in the case of a television signal processed through a video switcher and the like or a signal passed through recording or reproduction apparatus, for example, where sharp potential fluctuations are a particular problem. Further, in view of the fact that sync signals cannot be separated independently as described above, the sync detection of the receiver is effected first without clamp and a clamp is required to be effected immediately after sync pull-in, thus requiring a high-speed clamp at this moment.
As explained above, in a receiver of the television transmission system under consideration, a horizontal clamp which may be effected at high speed is liable to be fluctuated owing to noises at the time of deterioration of S/N, while as to the vertical clamp free of such a trouble but whose time constant cannot be shortened, it takes a considerable length of time before the rise immediately after power is thrown in or channel switching or clamp pull-in at the time of signal switching of a signal source other than for receiving a satellite broadcasting with a video switcher or the like. A most suitable clamp system has therefore been desired.